Haixi Miao

Astrocytes in glaucomatous optic neuropathy.

Glaucoma, the second most prevalent cause of blindness worldwide, is a degenerative disease characterized by loss of vision due to loss of retinal ganglion cells. There is no cure for glaucoma, but early intervention with drugs and/or surgery may slow or halt loss of vision. Increased intraocular pressure (IOP), age, and genetic background are the leading risk factors for glaucoma. Our laboratory and other investigators have provided evidence that astrocytes are the cells responsible for many pathological changes in the glaucomatous optic nerve head (ONH). Over several years, in vivo and in vitro techniques characterized the changes in quiescent astrocytes that lead to the reactive phenotype in glaucoma. Reactive astrocytes alter the homeostasis and integrity of the neural and connective tissues in the ONH of human and experimental glaucoma in monkeys. During the transition of quiescent astrocytes to the reactive phenotype altered astrocyte homeostatic functions such as cell-cell communication, migration, growth factor pathway activation, and responses to oxidative stress may impact pathological changes in POAG. Our data also suggests that the creation of a non-supportive environment for the survival of RGC axons through remodeling of the ONH by reactive astrocytes leads to progression of glaucomatous optic neuropathy.

Pressure induces loss of gap junction communication and redistribution of connexin 43 in astrocytes

Astrocytes, the major glia in the nonmyelinated optic nerve head (ONH), connect via gap junctions built of connexin‐43 (Cx43) to form a functional syncytium allowing communication and control of ionic and metabolic homeostasis of retinal ganglion cells (RGCs) axon. We examined gap junction intercellular communication (GJIC) by scrape loading assays in human ONH astrocytes exposed to hydrostatic (HP) or ambient pressure (CP) in vitro. Immunostaining, immunoprecipitation, and immunoblots were used to detect Cx43 distribution and phosphorylation in astrocytes exposed to HP with/without EGF receptor (EGFR) tyrosine kinase inhibitors AG1478 and AG82 and MAPK inhibitors U0126, PD98059, and SB203580. The data indicates that upon exposure to HP, astrocytes decrease GJIC and exhibit altered cellular localization and phosphorylation of Cx43. Inhibition of EGFR blocked the effects of HP on GJIC and HP‐induced Cx43 tyrosine phosphorylation. Inhibitors of MAPK‐ ERK1/2 and ‐p38 caused partial closure of GJIC under CP and HP, which was maintained for 6 h. Inhibition of Big Mitogen‐Activated Kinase 1/ERK5 (BMK1/ERK5) caused partial closure under CP and HP followed by full recovery after 6 h. Inhibition of MAPK did not affect the HP‐induced increase in Cx43 serine 279/282 phosphorylation. We conclude that activation of the EGFR pathway in response to HP leads to decrease of GJIC via tyrosine phosphorylation of Cx43 in ONH astrocytes. In glaucoma under conditions of elevated intraocular pressure (IOP), astrocytes may lose GJIC altering the homeostasis of RGC axons, adopting the reactive phenotype, contributing to glaucomatous neuropathy.

Susceptibility to glaucoma: differential comparison of the astrocyte transcriptome from glaucomatous African American and Caucasian American donors.

BackgroundEpidemiological and genetic studies indicate that ethnic/genetic background plays an important role in susceptibility to primary open angle glaucoma (POAG). POAG is more prevalent among the African-descent population compared to the Caucasian population. Damage in POAG occurs at the level of the optic nerve head (ONH) and is mediated by astrocytes. Here we investigated differences in gene expression in primary cultures of ONH astrocytes obtained from age-matched normal and glaucomatous donors of Caucasian American (CA) and African American (AA) populations using oligonucleotide microarrays.ResultsGene expression data were obtained from cultured astrocytes representing 12 normal CA and 12 normal AA eyes, 6 AA eyes with POAG and 8 CA eyes with POAG. Data were normalized and significant differential gene expression levels detected by using empirical Bayesian shrinkage moderated t-statistics. Gene Ontology analysis and networks of interacting proteins were constructed using the BioGRID database. Network maps included regulation of myosin, actin, and protein trafficking. Real-time RT-PCR, western blots, ELISA, and functional assays validated genes in the networks.ConclusionCultured AA and CA glaucomatous astrocytes retain differential expression of genes that promote cell motility and migration, regulate cell adhesion, and are associated with structural tissue changes that collectively contribute to neural degeneration. Key upregulated genes include those encoding myosin light chain kinase (MYLK), transforming growth factor-β receptor 2 (TGFBR2), rho-family GTPase-2 (RAC2), and versican (VCAN). These genes along with other differentially expressed components of integrated networks may reflect functional susceptibility to chronic elevated intraocular pressure that is enhanced in the optic nerve head of African Americans.

Gene expression and functional studies of the optic nerve head astrocyte transcriptome from normal African Americans and Caucasian Americans donors

Purpose To determine whether optic nerve head (ONH) astrocytes, a key cellular component of glaucomatous neuropathy, exhibit differential gene expression in primary cultures of astrocytes from normal African American (AA) donors compared to astrocytes from normal Caucasian American (CA) donors. Methods We used oligonucleotide Affymetrix microarray (HG U133A & HG U133A 2.0 chips) to compare gene expression levels in cultured ONH astrocytes from twelve CA and twelve AA normal age matched donor eyes. Chips were normalized with Robust Microarray Analysis (RMA) in R using Bioconductor. Significant differential gene expression levels were detected using mixed effects modeling and Statistical Analysis of Microarray (SAM). Functional analysis and Gene Ontology were used to classify differentially expressed genes. Differential gene expression was validated by quantitative real time RT-PCR. Protein levels were detected by Western blots and ELISA. Cell adhesion and migration assays tested physiological responses. Glutathione (GSH) assay detected levels of intracellular GSH. Results Multiple analyses selected 87 genes differentially expressed between normal AA and CA (P<0.01). The most relevant genes expressed in AA were categorized by function, including: signal transduction, response to stress, ECM genes, migration and cell adhesion. Conclusions These data show that normal astrocytes from AA and CA normal donors display distinct expression profiles that impact astrocyte functions in the ONH. Our data suggests that differences in gene expression in ONH astrocytes may be specific to the development and/or progression of glaucoma in AA.

Modulation of Factors Affecting Optic Nerve Head Astrocyte Migration

PURPOSE The authors investigated the role of myosin light chain kinase (MYLK) and transforming growth factor beta (TGFbeta) receptor pathways in optic nerve head (ONH) astrocyte migration. They further investigated how the expression of these genes is altered by elevated hydrostatic pressure (HP). METHODS PCR was used to determine the isoforms of MYLK expressed in ONH astrocytes. siRNAs against MYLK (all isoforms) and TGFbeta receptor 2 (TGFBR2) were prepared and tested for effects on the migration of cultured ONH astrocytes. Finally, the effects of elevated HP (24-96 hours) on the expression of MYLK isoforms and selected TGFbeta pathway components were measured. RESULTS Multiple isoforms of MYLK are present in ONH astrocytes from Caucasian (CA) and African American (AA) donors. Both populations express the short form (MYLK-130) and the long form (MYLK-210) of MYLK and a splicing variant within MYLK-210. MYLK-directed siRNA decreased MYLK expression and cell migration compared with control siRNA. siRNA directed against TGFbeta receptor 2 also decreased cell migration compared with control and decreased extracellular matrix genes regulated by TGFbeta signaling. Elevated HP increased the expression of MYLK-130 and MYLK-210 in both populations of astrocytes. However, TGFbeta2 was uniquely upregulated by exposure to elevated HP in CA compared with AA astrocytes. CONCLUSIONS Differential expression of TGFbeta pathway genes and MYLK isoforms observed in populations of glaucomatous astrocytes applies to the elevated HP model system. MYLK may be a new target for intervention in glaucoma to alter reactive astrocyte migration in the ONH.

Differential Effects of Elevated Hydrostatic Pressure on Gene Expression and Protein Phosphorylation in Optic Nerve Head Astrocytes

1.1 Primary Open Angle Glaucoma (POAG) Elevated intraocular pressure (IOP) is the most important risk factor in POAG. Typically affecting older adults, in POAG the IOP exceeds the level that is tolerated by that individual’s optic nerve head (ONH). However, many individuals with clinical ocular hypertension do not exhibit glaucomatous changes in the optic disk; whereas, some individuals will develop glaucomatous changes at clinically normal IOP levels. These clinical findings indicate that individual variability in susceptibility of the ONH to IOP is an important factor in glaucomatous optic neuropathy. However, the molecular and cellular factors that may underlie variability in susceptibility of the ONH to elevated IOP have not been elucidated. The target of the mechanical stress generated by elevated IOP is the lamina cribrosa in the ONH (Bellezza et al., 2003). In the glaucomatous ONH, compression, stretching, and remodeling of the cribriform plates of the lamina cribrosa occur. In many POAG patients, these elevated IOP related changes result in remodeling of the extracellular matrix (ECM), altering the quantity and composition of several ECM macromolecules that significantly affect the biomechanical properties of the tissue supporting the nerve fibers (Hernandez, 2000; Bellezza et al., 2003). Alterations of ECM components of the ONH in glaucoma perhaps sets the stage for further optic nerve damage from IOP during the progression of the disease.